Even if you have a friendly attitude toward men and manhood and have not been confused by bad feminism or the post-modern posing of "it's all a performance", it's pretty difficult to come up with one man who embodies manhood completely, or even best.
Two thoughts on why.
First, masculinity takes different shapes throughout the life-cycle. When a boy transitions to manhood, it is to young manhood, as is proper. But that has to ripen all through life and keep changing. There is a "fit" for manhood that suits a twenty-five year old but that no longer fits when he is sixty. So manhood is a naturally moving archetype. You might choose a man in his prime or at his peak as your model of The Manliest Man, but it would leave out way too much.
Second, there is not just one kind of real man needed in nature. A soldier, a farmer, a laborer, an artist, an athlete, a brewery worker...My guess is that all real men have some basic things in common but the context in which they play them out, the shape of their particular characters, place, time, etc. makes for a variety of men, all of whom can lay claim to the title of "man".
The image I am using of late is a constellation. It takes several stars to make a constellation, but not all of them are equally bright or central. And not every star is part of this constellation, Orion: the great hunter of Greek mythology, which seems a good choice for
the image.